```
$ sox -n rp.mp3 synth 3 sine 500
$ ffmpeg -i rp.mp3 rp.ogg
$ ffmpeg -i rp.mp3 rp.m4a
$ mp3gain -s i rp.mp3
$ vorbisgain rp.ogg
$ aacgain rp.m4a
$ ffprobe rp.mp3
Input #0, mp3, from 'rp-i.mp3':
Metadata:
replaygain_reference_loudness: 89.0 dB
replaygain_track_gain: -4.220000 dB
replaygain_track_peak: 0.307712
MP3GAIN_MINMAX : 134,177
Duration: 00:00:03.02, start: 0.000000, bitrate: 69 kb/s
Stream #0:0: Audio: mp3, 48000 Hz, mono, fltp, 64 kb/s
Side data:
replaygain: track gain - -4.220000, track peak - 0.000007, album gain -
unknown, album peak - unknown,
$ ffprobe rp.ogg
Input #0, ogg, from 'rp.ogg':
Duration: 00:00:03.02, start: 0.000000, bitrate: 26 kb/s
Stream #0:0: Audio: vorbis, 48000 Hz, mono, fltp, 80 kb/s
Metadata:
ENCODER : Lavc58.35.100 libvorbis
REPLAYGAIN_TRACK_PEAK: 0.31471366
REPLAYGAIN_TRACK_GAIN: -4.28 dB
Side data:
replaygain: track gain - -4.280000, track peak - 0.000007, album gain -
unknown, album peak - unknown,
$ ffprobe rp.m4a
Input #0, mov,mp4,m4a,3gp,3g2,mj2, from 'rp.m4a':
Metadata:
major_brand : M4A
minor_version : 512
compatible_brands: isomiso2
encoder : Lavf58.20.100
replaygain_track_peak: 0.31
replaygain_track_gain: -4.22
replaygain_track_minmax: 117,148
Duration: 00:00:03.05, start: 0.000000, bitrate: 56 kb/s
Stream #0:0(und): Audio: aac (LC) (mp4a / 0x6134706D), 48000 Hz, mono,
fltp, 52 kb/s (default)
Metadata:
handler_name : SoundHandler
Side data:
replaygain: track gain - -4.220000, track peak - 0.000007, album gain -
unknown, album peak - unknown,
```
Seems like vorbisgain is the only one who read [the
doc](https://wiki.hydrogenaud.io/index.php?title=ReplayGain_specification):
- the metadata must be in upper case
- mp3 files should use ID3v2 (it's ape by default with mp3gain)
- the gain must be with a "dB" prefix
Additionally, [`liquidsoap` seem to require a leading space in the metadata
value](https://github.com/savonet/liquidsoap/blob/3350999c9efe447d8a226d59ffc061ce55004580/src/operators/amplify.ml#L47),
in contradiction with [the
doc](https://www.liquidsoap.info/doc-dev/reference.html#amplify).
The only way I see out of this madness, outside from fixing these tools/finding
spec-compliant ones, would be to make the ` dB` prefix optional and the
metadata name case insensitive. Since this is specific to `replaygain`, a new
`replaygain` operator could be created as a `amplify` wrapper.
Bonus point if this operator supports preamp and clipping prevention as
required by the spec. It could also be added as a note to use `amplify` in the
doc.
What do you think?
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